The invention relates to a method of making photonic crystals and passive components comprising photonic crystals. In particular, the method includes one or more extrusion steps to produce a cellular or channeled object followed by a step of viscously sintering the object. The sintered, channeled object is heated and drawn to a final diameter.
A photonic crystal is a structure having a periodic variation in dielectric constant. The periodic structure may be 1, 2 or 3 dimensional. The photonic crystal allows passage of certain light wavelengths and prevents passage of certain other light wavelengths. Thus the photonic crystals are said to have allowed light wavelength bands and band gaps which define the wavelength bands which are excluded from the crystal.
At present, the wavelengths of interest for telecommunication applications are in the range of about 800 nm to 1800 nm. Of particular interest is the wavelength band in the range of about 1300 nm to 1600 nm.
Light having a wavelength in the band gap may not pass through the photonic crystal. Light having a wavelength in bands above and below the band gap may propagate through the crystal. A photonic crystal exhibits a set of band gaps which are analogous to the solutions of the Bragg scattering equation. The band gaps are determined by the pattern and period of the variation in dielectric constant. Thus the periodic array of variation in dielectric constant acts as a Bragg scatterer of light of certain wavelengths in analogy with the Bragg scattering of x-rays wavelengths by atoms in a lattice.
Introducing defects into the periodic variation of the photonic crystal dielectric constant can alter allowed or non-allowed light wavelengths which can propagate in the crystal. Light which cannot propagate in the photonic crystal but can propagate in the defect region will be trapped in the defect region. Thus, a point defect within the crystal can serve as a localized "light cavity". Analogously, a line defect in the photonic crystal can act as a waveguide for a mode having a wavelength in the band gap, the crystal lattice serving to confine the guided light to the defect line in the crystal. A particular line defect in a three dimensional photonic crystal would act as a waveguide channel, for light wavelengths in the band gap. A review of the structure and function of photonic crystals is found in, "Photonic Crystals: putting a new twist on light", Nature, vol. 386, Mar. 13, 1997, pp. 143-149, Joannopoulos et al.
A first order band gap phenomenon is observed when the period of the variation in dielectric constant is of the order of the light wavelength which is to undergo Bragg scattering. Thus, for the wavelengths of interest, i.e., in the range of about 1300 nm to 1600 nm, as set forth above, a first order band gap is achieved when the period of the variation is about 500 nm. However, photonic crystal effects can occur in crystals having dielectric periodicity in the range of about 0.1 .mu.m to 5 .mu.m. A two or three dimensional photonic crystal having even this larger special periodicity is difficult to fabricate.
In U.S. Pat. No. 5,774,779, Tuchinskiy, a method of making multi-channeled structures is described. Rods are bundled together and reduced in diameter by extrusion. The step of bundling and extrusion may be repeated using rods which have already been extruded one or more times. However, no step of drawing is disclosed, so that channel density, expressed as number of channels per unit area, is not large enough to produce a photonic crystal.
There is a need for a method of making photonic crystals of two or three dimensions which is repeatable, versatile, and potentially adaptable to a manufacturing environment, as compared to that of a laboratory.